1. Field of the Invention
The present invention relates to a blue electroluminescent polymer and an organic-electroluminescent device using the same, and more particularly, to a blue electroluminescent polymer including a phenoxazine unit in a polyarylene backbone and an organic-electroluminescent device using the same that offers high luminance and efficiency.
2. Description of the Related Art
Since the report of a multi-layered organic-electroluminescent (EL) device, each layer having a specified function, by C. W Tang of EASTMAN KOKAK COMPANY, due to the advantages of lightweight, thinness, a wide range of colors, fast switching speed, and high luminescence at low driving voltage, many studies on organic-electroluminescent devices have been conducted over the last decade. As a result of the efforts, great improvements in the performance of multi-layered organic-electroluminescent devices have been accomplished in a short period of time: balanced charge injection due to multi-layered structures, color tunability and higher quantum efficiency by doping, etc. In addition, new electrodes made of an alloy are available for organic-EL devices.
Such organic-EL devices may be classified depending on the molecular weight of their materials and manufacturing processes: devices manufactured from low molecular weight compounds and device manufactured using large molecular weight compounds. Low molecular weight compounds may be layered by vacuum deposition and may be easily purified to a high degree. In addition, color pixels may be easily obtained in a low molecular weight device. Despite the advantages of low molecular weight organic-EL devices, there still remain further improvements for practical application, for example, in quantum efficiency and color purity, and a need to prevent crystallization of thin layers. Various studies on such EL displays using low molecular weight compounds have been actively undertaken, especially in Japan and the U.S.A. For example, IDEMITSU-KOSAN CO., LTD. of Japan first exhibited in 1997 a 10-inch full color organic-EL display using a color-changing medium. PIONEER CORPORATION of Japan presented a 5-inch passive matrix (PM) full color organic-EL display. Recently, PIONEER CORPORATION and MOTOROLA INC. have arrived at an agreement concerning the mass production of cellular phones with an organic-EL display, implying that low molecular weight EL displays will be commercially viable in the near future.
Research on EL devices using polymers has been accelerated since the Cambridge Group's report in 1990 on the ability of poly(1,4-phenylene vinylene)(PPV), π-conjugated polymer, to emit light when exposed to electricity. π-conjugated polymers have an alternating structure of single bonds (σ-bonds) and double bonds (π-bonds), where π-electrons are evenly distributed to be freely movable in the polymer chain. Accordingly, π-conjugated polymers have semiconducting properties and may emit light of a visible range corresponding to the HOMOLUMO energy bandgap, via proper molecular designing, when applied to an emissive layer of an EL device. Such a polymer may be formed easily as a thin layer in the manufacture of EL devices, by spin coating or printing, at low costs and has a high glass transition temperature that allows the thin layer effective mechanical properties. Thus, such polymer-based EL devices are expected to be more commercially competitive than low molecular weight EL devices in the near future.
However, blue EL devices using polymers have problems of low color purity, high driving voltage, and low efficiency. To address the problems of polymer-based EL devices, there have been active studies. As an example, copolymerization (U.S. Pat. No. 6,169,163) or blending (Synthetic Metal, Vol. 106, pp. 115–119, 1999) of a fluorene-containing polymer was suggested for improved EL properties. However, additional improvements still remain to be made.